The overall goal of the proposed experiments is to determine the molecular mechanisms involved in the regulation of slow skeletal muscle contraction by troponin T (TnT) and to determine what regions of TnT are important for its physiological function. To accomplish this overall goal we will carry out two specific aims: SPECIFIC AIM 1. THE ROLE OF SLOW SKELETAL TROPONIN T ISOFORMS IN THE REGULATION OF MUSCLE CONTRACTION. Essentially nothing is known about the regulation of slow skeletal muscle by troponin T (TnT) and this is the prime focus of our study which will thoroughly investigate the function of slow skeletal muscle TnT (SSTnT) in the regulation of striated muscle contraction. The main hypothesis to be tested here is that the various N-terminal SSTnT isoforms modulate Ca2+- sensitivity and the relaxation properties of slow skeletal muscle contraction. We will answer the following questions: 1) Is the Ca2+-sensitivity of force development and/or ATPase activity affected by the N- or Cterminal regions of SSTnT? 2) Do SSTnT isoforms directly affect the affinity of TnC for Ca2+ or is there an indirect effect of SSTnT on the Ca2+ sensitivity of contraction (e.g., thin filament or crossbridge effect)? 3) Do the different SSTnT isoforms interact differently with tropomyosin? 4) Do SSTnT isoforms affect the activation and relaxation of force in skinned muscle fibers? 5) Are there other isoforms of SSTnT that have not been previously described? These studies will determine the role of the N-and C-terminal alternatively spliced regions of SSTnT on slow skeletal muscle contraction. SPECIFIC AIM 2. THE ROLE OF DIFFERENT REGIONS OF SLOW SKELETAL TROPONIN T IN THE PHYSIOLOGICAL FUNCTION OF TROPONIN T. To understand the role of different regions of TnT we will carry out the following: 1) Characterization of the region of SSTnT that is important for Ca2+-independent ATPase activity. 2) Characterization of the region(s) of SSTnT that interacts with SSTnl, CTnC (same isoform in slow skeletal and cardiac muscle) and tropomyosin. 3) Investigation of the importance of SSTnl in the function of SSTnT. No functional studies on any region of SSTnT have so far been reported. All the Specific Aims listed above focus on gaining a more detailed understanding of the role of SSTnT in slow skeletal muscle contraction, including the molecular mechanisms of SSTnT-linked activation of muscle contraction.